1. Field of the Invention
The present invention relates to colored polymer particles suitably used as a toner for electrophotography, electrostatic recording, electrostatic printing and the like, and to a method for effectively producing the colored polymer particles, and a toner and a developer using the colored polymer particles.
2. Description of the Related Art
As electrophotography many methods are known (see U.S. Pat. No. 2,297,691 and Japanese Patent Application Publication (JP-B) No. 42-23910). Generally, the method includes a electrostatic image forming step of electrically forming a latent image on a photoconductive layer containing a photoconductive material by various units, a developing step of developing the latent electrostatic image using a toner so as to form a toner image, a transferring step of transferring the toner image to a recording medium such as paper, a fixing step of fixing the transferred toner image onto the recording medium by heat, pressure, hot-press or solvent steam, a cleaning step of removing the residual toner on the photoconductor layer.
It is required that toners for electrophotography be produced by more energy-saving and environment-friendly methods. Conventionally, a melt-kneading and pulverization method has been known in current toner production methods. In recent years, chemical methods in a liquid solvent (for example, polymerization methods such as emulsion polymerization method, suspension polymerization method, dispersion polymerization method, dissolution and suspension method, and dissolution, suspension and extension method) have been used in most cases for production of toners. In view of recent environmental problems, as chemical toners produced through these chemical methods (polymerization methods), chemical toners termed as “capsule toner”, “core shell toner”, etc. are available that are provided in a form that makes efficient provision of desired functions possible.
In such toner production methods that use the melt-kneading and pulverization method, how uniformly each constituent material is dispersed and pulverized is important to ensure that the resultant toner particles have uniform shapes. In general, pulverized toner particles have irregular shapes with randomly-sized cross sections, and control of the shape or structure of toner particles become very difficult. Moreover, when coloring materials, releasing agents, charge controlling agents and the like are added to the toner, these additives tend to migrate to the surfaces of toner particles during a pulverization process because they cleavage along their crystal plane, causing problems that quality such as toner properties (e.g., flowability and charge ability) may be easily decreased, for example, variations may occur in the charge ability among individual toner particles.
Moreover, materials for toner are also limited in selection. That is, a resin colorant dispersion should be brittle enough to be pulverized. However, in case where the brittle resin colorant dispersion is used, particles having a wide range of a particle size are easily formed when the dispersion is pulverized at high speed. Particularly, a new problem occurs that a relatively large proportion of excessively pulverized particles is contained among particles. Moreover, highly brittle materials are not preferably used, because they are further pulverized easily when used in copiers or the like.
On the other hand, to overcome the problems of toner by these pulverization method, for example, a suspension polymerization method (one of the production method of chemical toner) is proposed in Japanese Patent Application Laid-Open (JP-A) No. 36-10231. In the suspension polymerization method, a polymerizable monomer, colorant, polymerization initiator, charge controlling agent and the like are dissolved or dispersed so as to produce a monomer composition, and then the monomer composition is dispersed and polymerized simultaneously in water containing a dispersion stabilizer, thereby to obtain a toner having a desired particle diameter.
However, the toner obtained by the method has an extremely wide particle size distribution and needs a classification step, thereby decreasing in yield. Particularly, this disadvantage outstands upon obtaining a toner having a smaller particle diameter, i.e. of 3 μm to 7 μm, which is demanded in recent years.
To solve these problems, JP-A Nos. 61-273552 and 62-73276 propose a method of nonaqueous dispersion polymerization in which polymerization is performed in the presence of a pigment in a dispersion medium to which a vinyl monomer is soluble and polymerized particles thereof are insoluble.
However, the method is not at all thought out to finely disperse the pigment to be compounded in the polymerized particles, and the pigments aggregate in polymerization system or a free pigment exists other than the polymer particles. As a result, a capsulation of the pigment within the polymer particles has been extremely difficult and the method is poor in practical use. Moreover, to solve these problems, Japanese Patent (JP-B) No. 2633383 discloses that in a polymerizable monomer, a pigment dispersion paste is prepared using a pigment and a pigment dispersant, and then dispersion-polymerized to obtain colored particles. As the surface of the colored particles becomes hydrophilic by using a hydrophilic solvent, when the colored particles used in a toner, a problem occurs in rise of charge, charge at high temperature and high humidity decreases, and charge variation increases with time. Many other problems occur, for example, nonuniform coloring, poor coloring ability, presence of free pigment, and wider particle size distribution. Moreover, depending on kinds of pigments (pigment dispersants), a desired toner polarity (positive or negative) is not easily obtained, the toner charge ability is easily adversely affected, and aging characteristics and environmental property are easily changed. Thus far, it is a reality that a satisfactory toner has not been obtained.
To solve the above-mentioned problems, JP-B Nos. 3558984 and 3537913 each proposes a chemical toner having a narrow particle size distribution by the dispersion polymerization method. However, after dispersion polymerization, the polymer particles may not be satisfactorily colored, and the resultant toner is poor in charge ability, aging characteristics and environmental property. Therefore, a satisfactory toner cannot be obtained at present.
Moreover, the method for producing a chemical toner can produce a toner having smaller diameter and narrower particle size distribution than those of a toner obtained by the pulverization method. However, the method causes problems, such as a large amount of waste liquid generated in production and requirement of a filtration step and a dry step and enormous amount of dry energy, leading to high cost, and also many other problems to be solved from the standpoint of global environment load and resource saving.
Particularly, conventional chemical toners are often granulated in water or a hydrophilic solvent, so that the toner surface tends to be hydrophilic, causing a decrease in charge ability, destabilization of aging characteristics and environmental property, and further inducing serious problems, such as developing and transferring failure, toner scattering and decrease in image quality.
As a method of solving these problems, JP-A No. 2007-047752 discloses a technique which does not use an organic solvent, not generate water and waste liquid, and not need dry energy, wherein as a dispersion solvent, a supercritical fluid or subcritical fluid is used instead of conventionally used water or an organic solvent. By using the supercritical fluid or subcritical fluid as a dispersion solvent, a toner having excellent toner properties such as charge ability, environmental property and stability over time and containing no residual monomer can be produced at low cost with less environmental load without generating waste liquid and performing a dry step.
Conventionally, in dispersion polymerization, when a colorant is previously added in a monomer and then subjected to reaction, reaction interference may occur or polymer particles may not be produced. Thus, polymer particles are colored after the reaction. When the supercritical fluid or subcritical fluid is used as a dispersion solvent, a colorant is added after polymer particles are produced. In this case, the particles are not satisfactorily colored or only the particle surface is colored. As a result, a toner having low coloring ability (low image density) and a poor weather-resistant toner are easily obtained.
Therefore, in the chemical toner produced by the conventional method, a toner having excellent toner properties, such as image density, charge and changes over time, is produced at low cost without generating waste liquid and performing a dry step, and a method for producing the toner have not been provided at present.